The long-term objectives are to identify the optical signals that specify ocular defocus and control reflex accommodation and the process of emmetropization, and to identify the mechanisms that mediate the optical signals. Two hypotheses will be tested: First, that accommodation responds to the vergence of light. Three experiments will examine the possibility that accommodation responds to the angle of incidence of light from the edges of the pupil. The two-dimensional Stiles-Crawford (S-C) effect will be measured and special "apodizing" filters will be created to neutralize the S-C effect, and to reverse the decentration of the S-C peak from the nasal to the temporal side of the pupil. Then accommodation will be driven (1-3 D; 0.2 Hz) with the S-C effect normal, neutralized and reversed. In the second experiment subjects will view a blurred monochromatic vertical or horizontal luminance edge through a pinhole pupil imaged 1.5 mm left, right, above or below the visual achromatic axis of the eye. Orientation of the edge will alternate between light/dark and dark/light at 0.2 Hz during trials lasting 40.96 seconds to simulate changes between myopic and hyperopic defocus. A third experiment will examine accommodation to a vertical monochromatic luminance edge (550 nm) that steps randomly toward or away from the eye (+/- 1.5 D) while an adapting field (550 nm) enters the eye through the nasal or temporal side of the pupil. Two target orientations (dark/light or light/dark) and two adaptation directions (nasal or temporal) will be used to determine whether channels' sampling from opposite sides of the pupil, mediate the response to myopic or hyperopic defocus. The second hypothesis is that the chromatic mechanism that uses longitudinal chromatic aberration to drive accommodation displays "defocus constancy" under diverse conditions of illumination, perhaps by comparing normalized cone-contrasts rather than by comparing cone-excitations. Accommodation will be monitored under open-loop conditions (pinhole) while myopic and hyperopic defocus is simulated by yellow/black gratings composed of red and green grating components with the same spatial frequency (4c/deg) but different contrasts. The yellow/black grating simulations will be illuminated by various combinations of red and green light. A final experiment will determine whether S-cone signals simply add to L- and M-cone signals, or whether S-cone signals are compared with L- or M-cone signals thus using chromatic aberration to determine defocus.